Embodiments of the inventive concept relate generally to semiconductor memory devices. More particularly, embodiments of the inventive concept relate to nonvolatile memory devices and related programming methods.
Semiconductor memory devices can be roughly divided into two categories based on whether they retain stored data when disconnected from power. These categories include volatile memory devices, which lose stored data when disconnected from power, and nonvolatile memory devices, which retain stored data when disconnected from power. Because nonvolatile memories retain stored data when disconnected from power, they are often used to store data that must be retained even when devices are powered down.
Examples of volatile memory devices include dynamic random access memory (DRAM) and static random access memory (SRAM), and examples of nonvolatile memory devices include electrically erasable programmable read only memory (EEPROM), ferroelectric random access memory (FRAM), phase-change random access memory (PRAM), magnetoresistive random access memory (MRAM), and flash memory.
In recent years, there has been an increase in the number of devices employing nonvolatile memory devices. As examples, nonvolatile memory devices are being used increasingly in MP3 players, digital cameras, cellular phones, camcorders, flash cards, solid state drives (SSDs), to name but a few. In addition, there has also been an increase in the overall storage capacity of nonvolatile memory devices, resulting in a tremendous amount of nonvolatile data storage in use today.
Among nonvolatile memories, flash memory has become an increasingly popular choice for many devices, due at least to its relatively high integration density, performance, and durability. Flash memory is commonly used, for instance, in computers and memory cards, as well as many other devices.
Most flash memories are classified as either NOR type or NAND type based on memory cell characteristics and the arrangement of bitlines and wordlines. NOR flash memory comprises multiple cell transistors connected in parallel to a bitline; NOR flash memory stores data using channel hot electron injection and erases stored data using Fowler-Nordheim (F-N) tunneling. NAND flash memory comprises multiple cell transistors connected in series to a bitline; NAND flash memory stores and erases data using F-N tunneling.
Memory cells in a flash memory typically store or more bits of data each. A flash memory cell capable of storing one bit of data has two threshold voltage states—one state representing a logical “1” and another state representing a logical “0”. A flash memory cell capable of storing two bits of data has four threshold voltage states representing logical “11”, “10”, “00”, and “01”. Similarly, a flash memory cell capable of storing three bits of data has eight threshold voltage states representing logical “111”, “110”, etc. Recently, various techniques have been studied for storing more than 4-bit data in one memory cell.